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Old 02-04-2019, 11:10 AM
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Ok, well I just outlined my procedure and it has worked for me. Just an option for others to look at and make their own decisions
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Old 02-04-2019, 12:31 PM
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Quote:
Originally Posted by dshriver View Post
I can run the math for you. I need to know the distance from the main gear to the desired CG. Based on your numbers I get a CG of 9.4" behind the main gear.
I'm just curious how you calculated a number of 9.4 inches behind the main gear?

Based on his numbers when I plug it in, I get a CG position of 28.50 (28.4963) inches from the tip of the spinner or 8 (7.9963) inches from the Mains

What formula are you using?
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Old 02-04-2019, 01:36 PM
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This is an interesting discussion. And Curiosity Killed the Cat. So I experimented FWIW.

With my unfinished fuselage, just 2 servos in the tail I did some weighing. First fuselage with main gear on floor for all readings, scale on floor with tail wheel on scale. Fuselage not level.

250g 8 7/8 0z.


Next, fuselage level. Scale on adjustable table, tail wheel on scale.

282g 10 oz.

Next, fuselage level. Scale on floor, wooden prop to level fuselage, scale reading zeroed for tare weight, tail wheel on wooden prop.

283g 10 oz.

Interesting.
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Old 02-04-2019, 02:21 PM
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OK, I measured once again after adding 5 pounds to the forward fuse.


Weight # Dist inch
Main Gear 1 13.8 21.5
Main Gear 2 14.5 21.5
Tail Gear 3.0 84
CG 31.3 27.49 Current Balance Point from Tip of Spinner
27.50 Proper Balance Point from Tip of Spinner

I talked to Wendell Hostetler and he did not recall adding any weight using the same setup to get it to balance. I simply cant believe I need to add 5 pounds of weight to achieve proper balance with an all up weight of 31 pounds.
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Old 02-04-2019, 02:40 PM
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here's my question...how does it balance at 30% MAC? You have a constant chord wing which makes locating 30% of chord very easy, ( wing chord x .3 = distance from leading edge ) and that's a good starting point and should be fine for a maiden.
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Old 02-04-2019, 03:27 PM
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Quote:
Originally Posted by myfivezees View Post
OK, I measured once again after adding 5 pounds to the forward fuse.


Weight # Dist inch
Main Gear 1 13.8 21.5
Main Gear 2 14.5 21.5
Tail Gear 3.0 84
CG 31.3 27.49 Current Balance Point from Tip of Spinner
27.50 Proper Balance Point from Tip of Spinner

I talked to Wendell Hostetler and he did not recall adding any weight using the same setup to get it to balance. I simply cant believe I need to add 5 pounds of weight to achieve proper balance with an all up weight of 31 pounds.
Is there anything you may have done in the build to add tail weight? Given the moment arm of the tail very little added here could result in a massive amount to the front to counter it.

Where is the recommended CG measurement point? I guess an alternative is to use this other unit that I also have used the EZ Balancer 2
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Old 02-04-2019, 07:23 PM
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unless you are committed to the idea that the CG is a fixed point and you have to balance at that point, on a plane that size, supporting the wing tip with an outstretched finger is fine.
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Old 02-06-2019, 12:45 PM
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Quote:
Originally Posted by myfivezees View Post
I recently purchased 3 postal scales to find and adjust the CG on my Hostetler RV-4.

I have used the spreadsheets I found here and elsewhere and they are telling me I need 5 pounds of nose weight to achieve proper balance. This just does not seem right.

The distance from the tip of the spinner to the main gear is 21.5".

The distance from the tip of the spinner to the tail wheel is 84"

Lear Main Weight is 11.6 lbs.
Right Main Weight is 12.2 lbs.
Tail wheel Weight is 3 lbs.


Does any know a mathematical formula to calculate what the weight should be on each point to achieve proper CG ?
Hello MFZ's

I have a Xicoy CG balancer and also a spreadsheet that I created where the spreadsheet numbers exactly match the Xicoy numbers.
Using the numbers you first posted, the span from the front mains to the tailwheel is 62.5".
The critical number I do not have is the distance from the front mains to the manufactures recommended CG point. If you would supply me with that number I will plug the info into my calculator. If you do not have that number, let me know the distance from the front mains back to the wing tube.
As calculated now the (actual CG) of the airplane is 7.00 inches behind the front mains. Remember the actual CG measurement may or may not be the correct balance point based on other critical input. However, if your manufactures CG does fall 7 inches behind the front mains, then your plane is exactly balanced. I suspect the correct CG balance point will fall about 6 inches behind front mains. If it does then your plane will be considered tail heavy.

Jaketab
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Old 02-06-2019, 01:31 PM
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It's all in the numbers, correct one's anyway.
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Old 02-10-2019, 06:05 PM
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Finally had time to get back to work on the RV-4.

I added about 2.8 pounds to the nose and the numbers came out as follows.

Distance to Main Gear: 21.5"
Distance to Tail Wheel: 84.0"

Right Main Weight: 12.72 lbs.
Left Main Weight: 13.33 lbs.
Tail Wheel Weight: 2.79 lbs.
Total Weight: 28.84 lbs.

Calculations indicate CG is 27.55" behind spinner bolt.
Recommended CG is 27.50" behind spinner bolt.

So, just a bit tail heavy.
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Old 02-11-2019, 03:26 PM
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Quote:
Originally Posted by myfivezees View Post
Finally had time to get back to work on the RV-4.

I added about 2.8 pounds to the nose and the numbers came out as follows.

Distance to Main Gear: 21.5"
Distance to Tail Wheel: 84.0"

Right Main Weight: 12.72 lbs.
Left Main Weight: 13.33 lbs.
Tail Wheel Weight: 2.79 lbs.
Total Weight: 28.84 lbs.

Calculations indicate CG is 27.55" behind spinner bolt.
Recommended CG is 27.50" behind spinner bolt.

So, just a bit tail heavy.
Looks good to me. My calculation indicates the plane is tail heavy by 0.02 lb. Subtract that amount from the tail or add slightly more to the to the front.
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Old 02-11-2019, 04:44 PM
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Thanks Jaketab. I appreciate the confirmation. Makes me feel a little more confident.
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Old 02-11-2019, 05:57 PM
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Post #6 in this thread on RCGroups is what I use and it works great.

The plane has to be level as if it was flying straight and level.

https://www.rcgroups.com/forums/show...cale-Airplanes
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Old 02-12-2019, 07:38 PM
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Quote:
Originally Posted by Jaketab View Post
Looks good to me. My calculation indicates the plane is tail heavy by 0.02 lb. Subtract that amount from the tail or add slightly more to the to the front.
So if my conversion calculation is correct, .02 lbs. = .32 oz. or about 3/10 of an ounce.
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